This invention relates generally to the field of film thickness measurement and film etching and deposition end point detection, and more specifically, to the field of film measurement and end point detection in a noisy environment, such as CMP processing of semiconductor wafers.
Many industrial processes require precise control of film thickness. In semiconductor processing, for example, a semiconductor wafer is fabricated in which one or more layers of material from the group comprising metals, metal oxides, insulators, silicon dioxide (SiO.sub.2), silicon nitride (SiN), polysilicon or the like, are stacked on top of one another over a substrate, made of a material such as silicon. Often, these layers are added through a subtractive process in which a layer having a thickness greater than the desired thickness is added to the previous layer, and then polished or ground down to the desired thickness through a process known as chemical mechanical planarization (CMP). The level of precision which is required can range from 0.001 .mu.m (a few atoms) to 100 .mu.m (about the thickness of a human hair).
With reference to FIG. 1, a CMP station 1 is illustrated. The station comprises upper platen or pallet 2 and lower platen or pallet 6 configured to move relative to one another. In the specific example illustrated in the figure, platen or pallet 2 is configured to axially move in a counter-clockwise direction relative to platen or pallet 6, and pallet 6 is configured to move horizontally or longitudinally relative to pallet 2. The upper pallet 2 has a mount 3 for fixably mounting semiconductor wafer 4. The lower pallet 6 is configured with a polishing pad 5, which in operation comes into contact with a layer 4a on the underside of wafer 4. A means 7 is provided for delivering a slurry to the upper surface 5a of the polishing pad 5. As shown, the delivery means includes one or more channels, one of which is identified in the figure with identifying numeral 7a, for delivering the slurry to the upper face 5a of the polishing pad. The slurry facilitates polishing of the wafer face, and typically comprises an abrasive chemical composition such as an ultra-fine grit formed of solid alumina or silica particles in a solution.
In operation, the upper and lower platens are moved relative to one another such that the upper surface of the polishing pad 5a frictionally engages the lower surface of the layer 4a of the wafer 4 while slurry is delivered to the upper surface of the polishing pad 5a through means 7. Through this procedure, polishing of the lower surface of layer 4a is accomplished. A mechanism (not shown) is provided to monitor the thickness of the layer 4a. This mechanism is used to control the CMP station so that the thickness of the layer 4a is not reduced below the desired thickness through excessive polishing. Exemplary embodiments of CMP stations configured to polish the surfaces of semiconductor wafers are described in U.S. Pat. Nos. 5,667,424; 5,663,797; 5,658,183; and 5,643,044, each of which is hereby incorporated by reference herein as though set forth in full.
In CMP processing, it is necessary to periodically if not continuously monitor film thickness in order to ensure that the thickness is not reduced below the desired amount though excessive polishing. Optical methods for monitoring film thickness in applications such as CMP have been proposed. According to such methods, an estimate of film thickness is derived through suitable analysis of light reflected from the surface of the film.
A problem with such methods as applied to CMP is that the monitoring of the film thickness cannot typically be done in in situ, i.e., while chemical mechanical planarization of the wafer is underway. The reason is that the slurry introduces a distortion into the reflected light, making it difficult if not impossible to accurately measure film thickness.
The problem is illustrated in FIG. 2, in which, compared to FIG. 1, like elements are identified with like identifying numerals. As shown, a slurry 8 is interposed between the lower surface of film 4a, and the upper surface 5a of polishing pad 5. Light 9 is directed to the film 4a for the purpose of measuring the thickness of the film. The reflected light 10 which ensues is captured and analyzed. As can be seen, the light 10 passes through the slurry 8. This process introduces distortion or noise into the light 10.
Consequently, the CMP procedure must typically be halted and the slurry removed from the film 4a in order to allow for accurate measurement of film thickness. Since monitoring of film thickness must be performed periodically if not continuously, the overhead involved in repeatedly stopping the CMP procedure, and removing the slurry, can be prohibitively expensive in terms of reduced throughput of the CMP procedure. In many cases, this renders the optical method of film measurement infeasible for application to CMP because of the overhead involved.
Another problem with such approaches is that distortion introduced by the optical components of the film measuring apparatus can adversely impact the accuracy of film thickness measurement. With reference to FIG. 2, each of the optical components involved in directing light 9 to the film 4a, and in capturing and analyzing the reflected beam 10, can introduce such a distortion into the reflected light 10.
Optical methods for film thickness measurement have been proposed which employ spectral or Fourier analysis of the light reflected from the film. Representative examples of such methods are described in U.S. Pat. Nos. 5,646,734; 5,642,196; 5,587,792; 5,227,861; 4,984,894; 4,555,767; 3,985,447; and 3,880,524. However, such methods are computationally complex, require time-consuming comparisons with theoretical or expected waveforms, employ excessive numbers of analytical steps to determine film thickness, or require time-consuming angular and mechanical movements or sweeps of an optical device such as a mirror. Consequently, in many cases, the process of monitoring film thickness employing such methods cannot be performed in real time, i.e., concurrently with and without requiring stoppage of CMP processing. In such cases, the rate of CMP processing must be slowed down to permit film monitoring. The result is a reduced throughput of CMP processing.
Accordingly, it is an object of the present invention to provide a method and apparatus for achieving rapid and accurate thin film measurement and end point detection in a noisy environment.
Another object is a method and apparatus for thin film processing which permits in situ monitoring of film thickness and end point detection during CMP processing.
Another object is an optical method and apparatus for thin film measurement and end point detection during CMP processing which provides for accurate measurement of film thickness despite distortion introduced by the slurry.
A further object is an optical method and apparatus for thin film measurement and end point detection which allows for accurate measurement of film thickness despite distortion introduced by the optical components thereof.
Another object is an optical method and apparatus for thin film measurement and end point detection which is capable of being operated in real-time during CMP processing.
A further object is an optical method and apparatus for thin measurement and end point detection which overcomes the disadvantages of the prior art.
Further objects of the subject invention include utilization or achievement of the foregoing objects, alone or in combination. Additional objects and advantages will be set forth in the description which follows, or will be apparent to those of ordinary skill in the art who practice the invention.